The present invention relates to a method of forming a buried interconnecting wire by filling a recessed portion formed in an insulating film, such as a groove, with a metal for interconnections in the process of manufacturing a semiconductor integrated circuit device or the like. In particular, it relates to technology for preventing the metal for interconnections from being oxidized or diffused into the insulating film.
At present, an aluminum alloy is used predominantly as a metal for interconnections in a semiconductor integrated circuit device or the like. On the other hand, copper or a copper alloy is receiving attention as a promising replacement for the aluminum alloy to be used in the next generation because of its lower resistivity and higher immunity to electromigration.
The largest problems presented by copper interconnections composed of copper or a copper alloy are the oxidation of the copper interconnections, the diffusion of copper from the copper interconnections into an insulating film, and poor processibility of a copper film, which remain to be solved before the copper interconnections are used in practice. In particular, the copper or copper alloy composing the copper interconnections is easily oxidized by and diffused into a SiO2 film used for an interlayer insulating film, which may adversely affect a device such as a transistor formed under the interlayer insulating film. To solve the problems, there has been proposed the formation of various barrier layers between the copper interconnections and the interlayer insulating film.
For example, Japanese Laid-Open Patent Publication HEI 02-240920 proposes a method of forming a barrier layer composed of a TiN film by performing N2 annealing with respect to a Cuxe2x80x94Ti alloy at a temperature of 800xc2x0 C. to prevent the oxidation and diffusion of copper.
On the other hand, Japanese Laid-Open Patent Publication HEI 06-275623 and xe2x80x9cDiffusion Barrier Properties of Transition Metals and Their Nitrides for Cu Interconnections (T. Nakao et. al, VMIC (1994))xe2x80x9d propose a method of forming a barrier layer composed of a tungsten nitride film by nitriding a W film by using a plasma in accordance with an ECR plasma method.
In the case of forming multi-layer metal interconnections by using the copper interconnections composed of copper or a copper alloy, the temperature of a heat treatment for a barrier layer should be 600xc2x0 C. or lower to prevent the oxidation and diffusion of copper in the underlying copper interconnections. However, a TiN film as formed by N2 annealing in accordance with the foregoing method cannot be implemented at a temperature of 600xc2x0 C. or lower. If the TiN film is formed by N2 annealing at a temperature of about 800xc2x0 C., on the other hand, the copper in the underlying copper interconnections may be oxidized and diffused disadvantageously. What results is the problem that the use of the copper interconnections is incompatible with the formation of the barrier layer by N2 annealing.
In the case of the latter method, the use of copper interconnections is compatible with the formation of the barrier layer by plasma nitriding, since the latter method allows the formation of a uniform barrier layer at a low temperature and hence is free from the problems of the oxidation and diffusion of the copper in the underlying copper interconnections. Unlike aluminum interconnections, it is extremely difficult to form copper interconnections from a copper film by performing dry etching with respect thereto, since a halogen compound is non-volatile.
To overcome the difficulty, there has been proposed a method of forming buried interconnections from copper by forming grooves in a region of an insulating film in which the interconnections are to be formed, depositing copper over the entire surface to form a copper film so that the copper is filled in the grooves, and removing portions of the copper film located outside the grooves.
In the case of forming the copper interconnections in accordance with such a method of forming buried interconnections, the barrier layer should be formed not only on the bottom of the grooves but also on the sidewalls thereof. In Japanese Laid-Open Patent Publication HEI 06-275623, as described above, the barrier layer composed of the metal nitride film is formed by depositing the metal on the bottom and sidewalls of the grooves to form the metal film and then plasma-nitriding the metal film under a pressure of 1 mTorr in accordance with the ECR plasma method.
However, the conventional method of forming the barrier layer composed of a metal nitride film by performing a plasma-nitriding process at 1 mTorr has such problems as illustrated in FIGS. 6(a) and 6(b). FIG. 6(a) illustrates the process steps of depositing a silicon dioxide film 12 on a silicon substrate 11, forming a groove 13 in the silicon dioxide film 12, depositing a tungsten film 15 over the entire surface of the silicon dioxide film 12, and then forming a tungsten nitride film 17 on the surface of the tungsten film 15 by a plasma-nitriding method. In this case, the mean free path of nitrogen ions at 1 mTorr is 10 cm or more, which is much larger than the sheath length (about 3 mm) of a sheath region between a plasma generation region and the silicon substrate, so that the nitrogen ions have an extremely low probability of colliding with nitrogen molecules in the sheath region. Accordingly, nitrogen ions 16 are incident upon the silicon substrate 11 in a direction substantially perpendicular thereto, as shown in FIG. 6(a). As a result, the nitrogen ions 16 seldom reach these portions of the tungsten film 15 covering the sidewalls of the groove 13, where a nitriding reaction does not proceed, so that the tungsten nitride film 17 is not formed on the sidewalls of the groove 13.
If a copper film 18 is deposited over the entire surface of the substrate with the tungsten nitride film 17 being not formed on the portions of the tungsten film 15 covering the sidewalls of the groove 13, copper contained in the copper film 18 is diffused into the silicon dioxide film 12 through the portions of the tungsten film 15 covering the sidewalls of the groove 13 because of unsatisfactory barrier property of the tungsten film 15, which adversely affects a device formed on the silicon substrate 11.
Although the foregoing process of forming the metal film by depositing a high-melting-point metal on the bottom and sidewalls of the groove is preferably performed by CVD which provides excellent coverage over the bottom and sidewalls of the groove, the following problem arises during the process: If the crystal growth of the high-melting-point metal is promoted to deposit a metal film having a low resistivity, undulations are formed on the surface of the metal film so that the plasma-nitriding process proceeds on some portions of the metal film, while stagnating on others, due to the presence of the undulations. Hence, a barrier layer composed of an equally nitrided metal nitride film cannot be obtained.
In view of the foregoing, it is therefore an object of the present invention is to ensure, when a conductive film made of a high-melting-point conductive material and formed with a recessed portion is to be nitrided by using a plasma, that a nitride film of the high-melting-point conductive material is formed even on the sidewalls of the recessed portion of the conductive film.
The present invention has been achieved based on the finding that, when a plasma-nitriding process is performed with respect to the conductive film made of the high-melting-point conductive material under a pressure of 10 Pa or more, nitrogen ions reach even the sidewalls of the recessed portion of the conductive film, resulting in positive nitriding of the conducive film.
A method of forming a buried interconnecting wire according to the present invention comprises: a first step of forming a first recessed portion in an insulating film deposited on a semiconductor substrate; a second step of depositing a high-melting-point conductive material on the insulating film to form a first conductive film composed of the high-melting-point conductive material and having a second recessed portion in a position corresponding to the first recessed portion of the insulating film; a third step of nitriding a surface of the first conductive film by using a plasma with the semiconductor substrate being held in a vacuum chamber maintained at a pressure of 10 Pa or higher to form a second conductive film composed of a nitride of the high-melting-point conductive material and having a third recessed portion in a position corresponding to the second recessed portion of the first conductive film; a fourth step of depositing a metal for interconnections on the second conductive film such that the metal for interconnections is filled in the third recessed portion of the second conductive film to form a metal film composed of the metal for interconnections; and a fifth step of removing the portions of the first conductive film, the second conductive film, and the metal film located outside the first recessed portion of the insulating film to form a buried interconnecting wire made of the metal for interconnections in the third recessed portion of the second conductive film.
According to the method of forming a buried interconnecting wire of the present invention, the following phenomenon is observed.
Since the mean free path xcexi is inversely proportional to the pressure P inside a vacuum chamber, the mean free path xcexi is 10 cm or more when the pressure P is 1 mtorr. When the pressure P is 10 Pa (75 mTorr), the mean free path xcexi of nitrogen ions becomes about 1 mm. Accordingly, a nitrogen ion passing through a sheath region having a sheath length Lsh of several millimeters collides with gas molecules (nitrogen molecules) on the average of several times.
In practice, the sheath length Lsh and the pressure P have a relationship represented by the following equation:
Lsh=bPxe2x88x92a (a,b:constants, 0 less than a less than 0.5)xe2x80x83xe2x80x83(1).
Since the sheath length Lsh becomes smaller as the pressure P becomes higher, the number of collisions in the sheath region is not simply inversely proportional to the pressure.
The sheath length Lsh is substantially irrelevant to the pressure P in an anode-coupled plasma processing apparatus which does not generate a cathode drop voltage VDC in a semiconductor substrate and in a plasma processing apparatus which applies no negative bias, since the constant a is substantially zero in these apparatus. In the case of employing the plasma processing apparatus, if the pressure P is 10 Pa or higher, the nitrogen ion collides with gas molecules on the average of several times in the sheath region, so that a large number of nitrogen ions do not collide with the semiconductor substrate in a direction perpendicular thereto (i.e., collide with the semiconductor substrate obliquely). In this case, the nitrogen ions incident obliquely upon the semiconductor substrate collide not only with the bottom of the recessed portion but also the sidewalls thereof, thereby promoting the reaction wherein the first conductive film made of a high-melting-point conductive material and formed on the sidewalls of the recessed portion is nitrided.
When the average number of collisions is 3 or more, the probability is that a majority of nitrogen ions collide with gas molecules at least once before reaching the semiconductor substrate.
As a result, substantially all the nitrogen ions travelling in a vertical direction toward the sample stage have their travelling direction changed to be incident obliquely upon the semiconductor substrate, so that a part of the nitrogen ions entering the third recessed portion of the second conductive film surely reach the sidewalls of the third recessed portion. Consequently, a nitride film of the high-melting-point conductive material is positively formed on the sidewalls of the third recessed portion of the second conductive film, which prevents the metal for interconnections from being diffused into the insulating film when it is composed of a silicon dioxide film.
In the method of forming a buried interconnecting wire, the third step preferably includes the step of nitriding the surface of the first conductive film by using a plasma with the semiconductor substrate being held in a vacuum chamber maintained at a pressure of 50 Pa or higher.
What results is the occurrence of such a phenomenon that the probability of a nitrogen ion colliding with a gas molecule immediately before bumping onto the semiconductor substrate becomes extremely high, since the nitrogen ion collides with gas molecules on the average of ten times or more in the sheath region under the pressure being maintained at 50 Pa or higher inside the vacuum chamber. Although the nitrogen ion has been accelerated in a direction perpendicular to the semiconductor substrate in the sheath region, when the nitrogen ion having energy of certain magnitude in a direction perpendicular to the substrate collides with a gas molecule immediately before bumping onto the semiconductor substrate, the energy of the nitrogen ion in a direction parallel with the semiconductor substrate is increased. Accordingly, the number of nitrogen ions incident upon the semiconductor substrate at a shallow angle with respect thereto is increased so that the nitrogen ions are more likely to reach the sidewalls of the third recessed portion of the second conductive film, thereby nitriding the portions more positively.
In the method of forming a buried interconnecting wire, the metal for interconnections in the fourth step is preferably copper or a copper alloy. This prevents the metal for interconnections, which is copper or a copper alloy, from being diffused in to the insulating film.
In the method of forming a buried interconnecting wire, the high-melting-point conductive material in the second step is preferably titanium, tantalum, or tungsten. This enables the formation of the barrier layer made of a nitride of titanium, tantalum, or tungsten.
In the method of forming a buried interconnecting wire, the third step preferably includes the step of nitriding the surface of the first conductive film by using a plasma with the semiconductor substrate being held at the ground potential or at a positive potential.
What results is the occurrence of such a phenomenon that nitrogen ions are accelerated at a potential equal to or higher than the plasma potential in a cathode-coupled plasma processing apparatus which generates a cathode drop voltage VDC in the semiconductor substrate or in a plasma processing apparatus which applies a negative bias to the substrate. In the plasma-nitriding process, the range of angles at which the nitrogen ions are scattered upon colliding with nitrogen molecules (scattering angle) is dependent on the energy of the ions, so that the ions are scattered in a wider range of angles as the energy of the ions is smaller. In the foregoing plasma processing apparatus wherein the nitride ions are accelerated at a potential equal to or higher than the plasma potential, therefore, it is difficult for the nitrogen ions to be incident upon the semiconductor substrate at such a small angle as to nitride the sidewalls of the second recessed portion of the first conductive film even after colliding with gas molecules. However, since the plasma-nitriding process is performed with the semiconductor substrate being held at the ground potential or at a positive potential, the nitrogen ions are accelerated only at the plasma potential, which widens the range of scattering angles of the nitrogen ions. As a result, the nitrogen ions are incident upon the semiconductor substrate at a small angle thereto, resulting in more equal formation of the second conductive film made of the nitride of the high-melting-point conductive material.
In the method of forming a buried interconnecting wire, the second step preferably includes the step of depositing the high-melting-point conductive material by CVD. This enables the formation of the first conductive film providing excellent coverage over the bottom and sidewalls of the first recessed portion of the insulating film.
When the second step is performed by CVD, the second step preferably includes the step of depositing the high-melting-point conductive material at a temperature within such a range as to suppress crystallization of the high-melting-point conductive material to form the first conductive film with at least a part thereof being amorphous.
As a result, the first conductive film is formed from the high-melting-point conductive material with at least a part thereof being amorphous, so that undulations are less likely to be formed on the surface of the first conductive film. In forming the second conductive film by plasma-nitriding the surface of the first conductive film, therefore, the nitriding process proceeds at a substantially constant rate over the surface of the first conductive film. This enables more equal formation of the second conductive film made of the nitride of the high-melting-point conductive material.